1. Field of the Invention
The present invention relates to an apparatus for controlling electric commonly used motors, such as DC motors, synchronous motors, induction motors and other motors.
2. Description of the Prior Art
FIG. 1 is a block diagram of a control system used in a three-phase synchronous motor and constructed in accordance with the prior art.
The control system comprises a position sensor 8 for sensing the rotational position P of a three-phase synchronous motor 7. The position sensor 8 is connected to a speed sensor 5 for sensing a rotational speed VL based on the rotational position P. The speed sensor 5 is connected to a subtracter 1 for subtracting the rotational speed VL from a speed command VLC to determine a speed error DVL. The subtracter 1 is also connected to a PID compensator 2 for compensating the speed error DVL to output a torque command value T. The PID compensator 2 is connected to a current command value setting means 3 for setting motor current commands CU, CV and CW based on the rotational motor position P and torque command value T. The current command value setting means 3 is also connected to a memory 6 for storing torque ripple error information in the electric motor and a power amplifier 4 for supplying three-phase currents IU, IV and IW corresponding to the motor current commands CU, CV and CW to the electric motor 7.
As shown in FIG. 2, the current command value setting means 3 comprises an address setting means 9 for preparing a read address of AD in the memory 6 based on the rotational motor position P, an adder 12 for adding torque error information MD read out of the memory 6 to the torque command value T to determine a current amplitude AM in the electric motor and a three-phase current setting means 10 for setting motor current commands CU, CV and CW from the current amplitude AM and rotational motor position P.
On operation, the position sensor 8 senses the rotational position P of the three-phase synchronous motor 7 while the speed sensor 5 senses the rotational speed VL from the rotational position P. On the other hand, the subtracter 1 receives the speed command VLC and subtracts the rotational speed VL from the speed command VLC to determine the speed error DVL. The speed error DVL is compensated by the PID compensator 2 which in turn outputs the torque command value T toward the current command value setting means 3. The current command value setting means 3 then sets the motor current commands CU, CV and CW based on the rotational motor position P and torque command value T. The motor current commands thus set are provided to the power amplifier 4 which in turn supplies the three phase motor currents IU, IV, IW to the electric motor 7.
The operation of the current command value setting means 3, that is, a process of preparing three phase current command values with a technique of compensating the torque ripple in the electric motor will be described below.
The rotational motor position P is used to prepare the read address AD of the memory 6 at which the torque error information in the electric motor is stored. The read address AD is then used to read the torque error information MD corresponding to the rotational motor position P. The adder 12 adds the torque error information MD to the torque command value T to calculate the current amplitude AM in the electric motor. The three phase current setting means 10 sets the motor current commands CU, CV and CW from the current amplitude AM and rotational motor position P according to the following equations: EQU CU=AMx sin P (1) EQU CV=AMx sin (P+120) (2) EQU CW=AMx sin (P+240) (3) EQU AM=T+MD (4).
The torque error information MD has been stored in the memory 6 based on the relationship between the rotational positions P (P1, P2 . . . ) and the corresponding torque errors (DT1, DT2 . . . ), as shown in FIG. 3. As a result, the slot torque ripple and other factors at each of the rotational motor positions can be compensated to realize a control by which the torque ripple being an error component in the motor output torque is relieved.
In general, the stator and rotator of the electric motor produce an error torque known as torque ripple since the shapes of the winding slots, magnetic poles and others are discontinuous in the rotational direction. The magnitude of such an error torque can vary between about 0.5% of the rated torque and equal to or higher than 10% from case to cases, depending on the type of motor. Particularly, a reluctance motor tends to increase the torque ripple since the magnetic resistance in the rotator becomes larger depending on the rotational position. Where it is required to control the electric motor with an increased accuracy, the torque ripple raises an important problem if the vibrations and noises in the electric motor may adversely affect the circumferences.
The motor control system of the prior art had a problem in that the torque and speed could not be accurately controlled, resulting from the dependency of the motor torque ripple on the rotational position as well as the torque; the non-linear change of the torque ripple depending on the magnitude of the torque; and the relation of cause and effect with the rotational speed from the delay in the control of the entire system. Particularly, where an electric motor having an increased reaction on the armature is used, the torque ripple form may not be in proportion to the output torque of the electric motor. A synchronous motor using an electromagnet has an increased freedom of an electromagnet field. However, even when such a type of synchronous motor is driven with an unsaturated electromagnet field density, the torque ripple form may not be in proportion to the output of the electric motor.
The structure of the electric motor itself has been modified to relieve the torque ripple therein. For example, the gap between the stator and the rotator has been increased; the structure of the rotator has been uniquely modified; and the stator or rotator has been arranged to be skewed relative to the rotational axis. However, such devises will reduce the efficiency in the electric motor. Thus, the motor must correspondingly be increased in size and complicated in structure, leading to increase of the manufacturing cost. Furthermore, the torque error in the electric motor depends on various parameters. If all the parameters such as torque compensation data, motor current values and others are to be stored in the memory means, the memory capacity will be huge, also leading to increase of the memory cost.